Mechanism of anomalous type electrodeposition of Fe-Ni alloys from sulfate solutions

The electrodeposition of Fe-Ni alloys was performed galvanostatically in the sulfate solutions of pH 1-3 at 40°C and the alloy deposition behavior was compared with that of Zn-iron-group metal alloys to investigate their codeposition mechanism. The deposition behavior of Fe-Ni alloy showed a typical feature of the anomalous codeposition, in which electrochemically less noble Fe deposits preferentially under most plating conditions. The anomalous codeposition behavior in Fe-Ni alloy deposition was evidently dependent on the pH buffer capacity of the solutions. This can be explained in terms of the preferential adsorption of FeOH on the deposition sites of more noble Ni due to the extremely smaller dissociation constant of FeOH⁺ than NiOH ^- in the multi-step reduction process of hydrated iron-group metal ions.